- Death of flower parts near bloom due to inflorescence necrosis (IN) is associated with high ammonium (NH₄⁺) concentration in flower clusters, shade, cool wet weather preceding bloom, and excessive vigor. Faults in NH₄⁺ assimilation are suspected to cause a rise in NH₄⁺ concentration to toxic levels in flower cluster but not other tissues. In this study, shading whole vines of Pinot noir increased flower NH₄⁺ concentration if applied at budbreak (early) or 1 wk before bloom (late), but only late application of shade to individual shoots raised it. Late, complete shoot defoliation raised flower cluster NH₄⁺ in all three years of the experiment. Early or late removal of lower leaves on a shoot also increased flower cluster NH₄⁺. Lamina, petiole, and other shoot tissue NH₄⁺ levels responded differently to treatments than flower clusters. IN severity was not affected by treatments; however, on shoots whose primary clusters had been removed at bloom secondary clusters did show significant differences, with IN being more severe in the early defoliation and late, lower defoliation treatments. Ethephon, sprayed on whole vines, slowed shoot growth to zero and increased IN severity greatly, however, flower cluster NH₄⁺ concentration was increased only 20% over the controls. Methionine sulfoximine applied as a cluster dip, increased flower cluster NH₄⁺ by 100%, yet resulted in little necrosis. Rootstock and clone affected Pinot noir flower cluster NH₄⁺ and IN severity. Rootstocks 420A caused lower and 101-14 and 3309 higher NH₄⁺ concentrations than the average. IN severity and flower cluster NH₄⁺ varied between vineyard sites, possibly due to environment and management differences. Pinot noir clones UCD23 and 32 had lower and UCD4 higher than average flower cluster NH₄⁺. IN in clones UCD4 and DJN115 was the least and UCD23 the most severe. A single 73kDa protein from grape shoot tissues reacted with anti-rice Fd-GOGAT IgG. An extraction method and assay for Fd-GOGAT activity from grape tissues was developed and gel filtration was used to show that the native enzyme is a dimer or trimer of the 73kDa protein. Activity was found in lamina, petiole, flower, rachis, and tendril, but not pedicel tissue.